Numerous earlier patents describe fluidistors, used with negative feedback, as volume flow meters. In U.S. Pat. No. 3,238,960, the fluid oscillator is described as a volume measuring element in combination with an orifice plate for measuring the flow in a pipe. In U.S. Pat. No. 5,127,173, a somewhat different embodiment is used, with a feedback fluidistor and a Venturi for creating a motive pressure that gives a proportional flow through the measuring element. Various methods for detecting pressure pulsations with frequency proportional to the pressure, are indicated in these patents and other patents in this technical field.
U.S. Pat. No. 5,083,452 discloses how the acoustic noise pattern in a pipe with a restriction, can be used to characterize the liquid in the pipe by means of statistical analysis methods (multivariate analysis).
However, fluidistor measuring devices have so far only been used in the form of external add-on units of a bulky type, and hence they have not been of interest for downhole applications in oil production fields. It is therefore of interest to improve this technology to achieve such an extension of the area of application, because with this technique it is possible to obtain good measurements, and it is not necessary to make great demands on the pressure sensor to be used.